miata2fast

iTrader: (2)

I am getting close to building a motor, and got some advice from Dave at Rebello Racing on what my clearances should be on the bearing journals.

His advice was:

Mains 0.0020" - 0.0022"
Rods 0.0018" - 0.0020"

This is somewhat conflicting of what I am hearing when I research the forum on journal clearances. Typically, I see rod journals having a larger gap, but I only looked at a few threads.

Could we start a discussion as to what journal clearances you are using for race applications (I guess street apps as well), and if you have different clearances between rods and mains, what is your reasoning.

miata2fast

iTrader: (2)

There has to be someone who has built some race motors here that have experience or opinions on this subject.

ScottFW

iTrader: (4)

No experience with a built motor yet, but it's in the cards so I'm interested in any discussion.

Stock clearances per the FSM are:

mains: .0008-.0014" (max .004")
rods: .0008-.0017" (max .004")

So Rebello's specs are a good bit looser than stock but still only about half of the recommended max. Presumably things turn a little more freely when they're more loosey goosey but you will need to make sure your oil pump can provide enough flow, probably run thicker oil, etc.

You may already know that ACL race bearings can be ordered with an extra .001" of oil clearance, same part number but with an "X" on the end. I've heard of people using both halves of the X, or just one half X and one half of the standard size depending if they want the full .001" or just .0005" more. I'd think it would be preferable to use these bearings where possible to get your desired clearances rather than machining the crank any more than absolutely necessary.

miata2fast

iTrader: (2)

I did not know that. Very interesting. Does that mean if I did that to the rod journals I get .0005" of stroke? Hot damn!

Is it true that you can only get a race bearing in standard and 10/10? I just learned that the hard way after I took a 10/10 crank to the machine shop to be inspected and machined. I am hoping there is enough meat there to get the crank cleaned up, run the clearance I need, and still be able to use a race bearing. I have a standard crank as well, but I was hoping to save it for my dream motor later.

I do not need anymore boat anchors.

AlwaysBroken

Stumbled across this thread and decided to add a data point.

Running ARP main studs and rod bolts with ACL bearing everywhere. Got an align hone for the mains. Mains are .0015, rods are .0012. Oil pressure is solid with 5w40, on a stage 2 BE pump.

edit: this is a mostly street driven turbo motor or I would have probably gone a bit looser. OP said he wanted to know about street setups so I posted.

icantlearn

following for knowlage

adryargument

iTrader: (1)

following, about to pull my motor

ctdrftna

iTrader: (1)

I think the clearances that you were given are pretty spot on. A good rule of thumb is .001" clearance per inch of journal diameter. So for a 1.5" rod journal you would want .0015" clearance. Like ScottFW said, you can mix and match std and x bearings to get your desired clearance as long as you keep all the x or all the std on the same side.

so something like this for your mains

1. 2. 3. 4. 5
std std. x. std. x
x. x. x. x. x.